Effects of controlled vortex generation and interactions in transverse jets

This experimental study examined the effects of controlled vortex generation and interactions created by axisymmetric excitation of a transverse jet, with a focus on the structural and mixing characteristics of the flow. The excitation consisted of a double-pulse forcing waveform applied to the jet, where two distinct temporal square-wave pulses were prescribed during a single forcing period. The two distinct pulses produced vortex rings of different strength and celerity, the strategic selection of which promoted vortex ring interactions or collisions in the near field to varying degrees. Jet flow conditions corresponding to a transitionally convectively and absolutely unstable upstream shear layer (USL) in the absence of forcing, at a jet-to-cross-flow momentum flux ratio of J=10, and to an absolutely unstable USL at J=7, were explored for a jet Reynolds number of 1800. Acetone planar laser-induced fluorescence imaging was utilized to quantify the influence of different prescribed temporal waveforms. All forcing conditions enhanced the spread, penetration, and molecular mixing of the jet as compared to the unforced jet, though to differing degrees. Interestingly, when the jet was convectively unstable, forcing which promoted vortex collisions provided the greatest enhancement in molecular mixing, whereas the absolutely unstable jet produced the greatest enhancement in mixing when the vortex rings did not interact, with important implications for optimized jet control

Fundamental performance limitations of modulated and demodulated control systems

We consider feedback performance limitations for modulated and demodulated control systems whose base systems have non-minimum phase (NMP) zeros or unstable poles. We first derive a transfer function for the modulated system and then show how the poles and zeros of this function are related to those of the base system. We next analyse the behaviour of the poles and zeros, when the modulation frequency is varied. Bode and Poisson integral constraints for the modulated system are then considered. The effect of a base system delay is also discussed

The actively controlled jet in crossflow

This study quantifies the dynamics of actuation for the temporally forced, round gas jet injected transversely into a crossflow, and incorporates these dynamics in developing a methodology for open loop jet control. A linear model for the dynamics of the forced jet actuation is used to develop a dynamic compensator for the actuator. When the compensator is applied, it allows the jet to be forced in a manner which results in a more precisely prescribed, temporally varying exit velocity, the RMS amplitude of perturbation of which can be made independent of the forcing frequency. Use of the compensator allows straightforward comparisons among different conditions for jet excitation. Clear identification can be made of specific excitation frequencies and characteristic temporal pulse widths which optimize transverse jet penetration and spread through the formation of distinct, deeply penetrating vortex structures

Structural and mixing characteristics in actively controlled transverse jets

These experiments explore the effect of external excitation on gaseous transverse jet (TJ) structural and mixing characteristics, emphasizing axisymmetric jet forcing. Sinusoidal as well as single and multiple square wave pulses, the latter with variable amplitudes, are explored for a range of jet-to-crossflow momentum flux ratios J, spanning regimes2 of absolutely unstable upstream shear layers (J 10). The studies utilize acetone PLIF imaging of the jet, as done for unforced jets3. Axisymmetric forcing, irrespective of the waveform, can enhance cross-sectional symmetry of the TJ for convectively unstable conditions, but generally disrupts the usually symmetric counter-rotating vortex pair (CVP) observed for the absolutely unstable TJ. Conditions producing deeply penetrating, periodic vortical structures, such as square wave forcing at critical stroke ratios, increase jet spread, but do not always optimize molecular mixing. Creating multiple vortex structures of different strengths via multiple square pulses leads to enhanced interactions and accelerated vortex breakdown, potentially increasing mixing